There has been considerable interest in recent years in developing high energy density batteries with lithium-containing anodes. Lithium metal is particularly attractive as the anode active material of electrochemical cells because of its light weight and high energy density, as compared, for example, to anode active materials such as lithium intercalated carbon anodes, where the presence of non-electroactive materials increases the weight and volume of the anode, thereby reducing the energy density of the anode. The use of lithium metal anodes, or those comprising lithium metal, provides an opportunity to construct cells that are lighter in weight and have a higher energy density than cells such as lithium-ion, nickel metal hydride or nickel-cadmium cells. These features are highly desirable for batteries in portable electronic devices such as cellular telephones and laptop computers, as noted, for example, by Linden in Handbook of Batteries, 1995, 2nd Edition, chapter 14, pp. 75-76, and chapter 36, p.2, McGraw-Hill, New York, and in U.S. Pat. No. 6,406,815 to Sandberg et al., the respective disclosures of which are incorporated herein by reference.
Thin film battery design is particularly suitable for portable electronic devices because their light weight combined with high surface area electrodes allows high rate capability, as well as reduced current density on charging and/or shorter charge time. High rate means the battery is capable on discharging its complete capacity in 20 minutes (3C rate) or less (>3C rate). Several types of cathode materials for thin-film lithium batteries are known, and include sulfur-containing cathode materials comprising sulfur-sulfur bonds, wherein high energy capacity and rechargeability are achieved from the electrochemical cleavage (via reduction) and reformation (via oxidation) of sulfur-sulfur bonds. Examples of sulfur containing cathode materials for use in electrochemical cells having lithium or sodium anodes include elemental sulfur, organo-sulfur, or carbon-sulfur compositions.
Lithium anodes in nonaqueous electrochemical cells develop surface films from reaction with cell components including nonaqueous solvents of the electrolyte system and materials dissolved in the solvents, such as, for example, electrolyte salts and materials that enter the electrolyte from the cathode. Materials entering the electrolyte from the cathode may include components of the cathode formulations and reduction products of the cathode formed upon cell discharge. In electrochemical cells with cathodes comprising sulfur-containing materials reduction products may include sulfides and polysulfides. The composition and properties of surface films on lithium electrodes have been extensively studied, and some of these studies have been summarized by Aurbach in Nonaqueous Electrochemistry, Chapter 6, pages 289-366, Marcel Dekker, New York, 1999. The surface films have been termed solid electrolyte interface (SEI) by Peled, in J. Electrochem. Soc., 1979, vol. 126, pages 2047-2051.
Among the examples of nonaqueous electrolyte solvents for lithium batteries described by Dominey in Lithium Batteries, New Materials, Developments and Perspectives, Chapter 4, pp. 137-165, Elsevier, Amsterdam (1994) are dioxolanes and glymes. Members of the glyme family, including dimethoxyethane (DME), diglyme (diethylene glycol dimethyl ether), triglyme (triethylene glycol dimethyl ether), ethylene glycol diethyl ether (DEE), and diethylene glycol diethyl ether, are often listed as being suitable electrolyte solvents, for example in U.S. Pat. No. 6,051,343 to Suzuki et al., U.S. Pat. No. 6,019,908 to Kono et al., and U.S. Pat. No. 5,856,039 to Takahashi. Electrolyte solvents comprising dioxolane and glymes have been described for use in nonaqueous electrochemical cells with a variety of anodes and cathodes. For example, in U.S. Pat. Nos. 4,084,045 to Kegelman, 4,086,403 to Whittingham et al., 3,877,983 to Hovsepian, and 6,218,054 to Webber, dioxolane and dimethoxyethane (DME) comprise the electrolyte solvents. Nimon et al. in U.S. Pat. No. 6,225,002 describe battery cells with gel or solid state electrolytes which comprise glymes and less than 30% by volume of dioxolane.
For rechargeable lithium/sulfur (Li/S) cells there is a need for further enhancement of cell performance, for example through improvements in the electrolyte solvent system. Ideally cells should have high utilization at practical discharge rates over many cycles. Complete discharge of a cell over time periods ranging from 20 minutes (3C) to 3 hours (C/3) is typically considered a practical discharge rate. Cycle life is typically considered to be the number of cycles to the point when a cell is no longer able to maintain acceptable levels of charge capacity, such as 80% of the initial capacity of the battery.
As used herein, a “100% utilization” (also called “sulfur utilization”) assumes that if all elemental sulfur in an electrode is fully utilized, the electrode will produce 1675 mAh per gram of sulfur initially present in the electrode. Among the prior art references that discuss and teach performance in Li/S cells, including parameters such as sulfur utilization, discharge rates, and cycle life are the following: (1) Peled et al., J. Electrochem. Soc., 1989, vol. 136, pp. 1621-1625 which discloses that Li/S cells with dioxolane electrolyte solvent mixtures achieve a sulfur utilization of no more than 50% at discharge rates of 0.1 mA/cm2 and 0.01 mA/cm2; (2) U.S. Pat. No. 5,686,201 to Chu describes a Li/S cell with a polymeric electrolyte that delivers 54% utilization at 30° C. and a low discharge rate of 0.02 mA/cm2. At 90° C. a utilization of 90% at a discharge rate of 0.1 mA/cm2 was achieved; (3) U.S. Pat. No. 6,030,720 to Chu et al., which describes liquid electrolyte Li/S rechargeable cells with sulfur utilization of approximately 40% for more than 70 cycles at discharge rates of 0.09 mA/cm2 (90 μA/cm2) and 0.5 mA/cm2 (500 μA/cm2). Another example (Example 4) describes a sulfur utilization of 60% over more than 35 cycles but at the low discharge rate of 0.09 mA/cm2; (4) U.S. Pat. No. 5,919,587 to Mukherjee et al., which describes liquid electrolyte Li/S rechargeable cells with sulfur utilization of approximately 36% for more than 60 cycles at discharge rates of 0.57 mA/cm2; (5) U.S. Pat. No. 6,110,619 to Zhang et al., which describes liquid electrolyte Li/S rechargeable cells with sulfur utilization of approximately 38% for more than 100 cycles and 19% for more than 200 cycles at discharge rates of 0.33 mA/cm2; (6) U.S. Pat. No. 6,544,688 to Cheng, which describes liquid electrolyte Li/S rechargeable cells with sulfur utilization of approximately 45% for more than 100 cycles at discharge rates of 0.42 mA/cm2; and (7) U.S. Pat. No. 6,344,293 to Geronov, which describes liquid electrolyte Li/S rechargeable cells with sulfur utilization of approximately 21% for more than 275 cycles at discharge rates of 0.41 mA/cm2.
Among the prior art references that discuss and teach the effect of different glycol ethers in electrolytes on the performance of lithium cells are the following: (1) Nishio et al., J. Power Sources, 1995, vol. 55, pp. 115-117, which discloses that discharge capacities of MnO2/Li cells in electrolyte solvent mixtures of propylene carbonate (PC) with ethers DME, ethoxymethoxyethane (EME), or DEE (1:1 volume ratio) show declining capacity in the order DME/PC>EME/PC>DEE/PC; and (2) U.S. Pat. No. 5,272,022 to Takami et al., which discloses lithium ion batteries in which the electrolyte solvents include carbonates mixed with the glymes DME, DEE, and EME. The cycle life of cells with electrolyte solvent mixtures of DME with diethyl carbonate and propylene carbonate is greater than the cycle life obtained with EME and these carbonates. In summary, in these head-to-head comparisons DME containing electrolyte solvent mixtures outperform the equivalent EME containing solvent mixtures.
In U.S. Pat. No. 4,804,595 to Bakos et al. it is reported that 1,2-dimethoxypropane provides comparable performance to DME in electrolyte formulations with propylene carbonate in electrochemical cells with lithium anodes and MnO2 or FeS5 cathodes.